The aperture is the ratio of the focal lenght by the effective diameter of the lens - since the focal lenghts are the same, the opening of the f/2.8 lenses are twice the size of the f/4. Hence you're paying for more glass (i.e. a faster lens) + materials to keep it together:
The Canon 70-200 f/2.8 is almost twice the cost of the f/4 - IMO you're getting a bargain :idea:

-> Plus the Sigma comes with a fitted case, hood, and a quick release tripod mount - What is included with the f/4???